Heat oil in a 12-inch heavy skillet over medium heat until it shimmers.,Cook onion with 1/4 teaspoon salt and 1/8 teaspoon pepper, stirring occasionally, until golden, about 6 minutes.,Add spinach and cook, stirring occasionally, until tender and liquid spinach releases has evaporated, about 4 minutes.,Divide between 2 plates.,Pat turkey cutlets dry and season with 1/4 teaspoon each of salt and pepper (total).,Heat butter in skillet over medium heat until foam subsides, then sear turkey until pale golden, 1 to 3 minutes.,Flip cutlets over and place 1/2 slice prosciutto on each.,Top with cheese, then cover and cook until cheese is melted and turkey is just cooked through, about 1 minute.,Transfer turkey to plates with spinach.,______